1. Field of the Invention
Embodiments of the present invention generally relate to downhole production operations and particularly to measuring volumetric fractions of individual phases of a multiphase mixture.
2. Description of the Related Art
Producing oil and gas wells typically provide a multiphase mixture of oil, gas, and water. Real-time, downhole flow data regarding the mixture (e.g., flow rate of the mixture, individual phase flow rates and fractional phase volumes, also referred to as “void fractions”) is widely acknowledged to be of significant value for production optimization. This is particularly the case for high-cost deepwater developments and in complex multi-lateral wells. Downhole flow data may be utilized in various production operations.
As an example, in multi-zone completions, downhole flow data can be used to allocate production from individual zones. This can be achieved either with flow data gathered above each producing zone or with flow data gathered between each producing zone (using total flow measured at the surface) to obtain the contribution of the uppermost zone. As another example, flow data gathered from one or more locations along a lateral section of a horizontal well can help identify which parts of the lateral section are contributing to flow and may help locate a production anomaly, such as a water or gas breakthrough, thus allowing localized well stimulation or other well treatments to be performed to increase well productivity.
Real-time downhole flow rate data may also allow determination of a well productivity index at any time without need for intervention, determination of contribution of multiple zones in commingled production. Further, real-time downhole flow data may reduce the need for surface well tests and eliminate the need for associated equipment, such as a surface test separator, thereby reducing production costs.
Despite the potential value of real-time downhole multiphase flow data, the lack of equipment capable of reliable and continuous downhole multiphase flow measurements has severely limited its application. Equipment capable of withstanding harsh operation conditions (e.g., extreme temperatures and pressures), have typically been limited to absolute temperature and pressure sensors. Recently, however, a limited number of techniques have been developed to gather multiphase downhole flow data.
These techniques typically involve downhole mixture density measurements from complex meters (densitometers) that suffer a number of drawbacks, such as nuclear fluid densitometers (NFD). NFDs typically contain complex electronics for performing density measurements that may exhibit reliability and accuracy problems when subjected to downhole operating conditions. Further, because the nuclear densitometers have a radioactive source, due to environmental issues, approvals may be required, and operating personnel may require extensive training.
As an alternative to NFDs, other types of flow meters may be used, such as a Venturi flow meters, designed to correlate measured pressure differences with density measurements to predict individual phase flow rates. However, these devices suffer from a number of disadvantages including restricted access below the device (which may prevent the running of tools below the device) and significant pressure loss due to the restrictive nature of the device. Further, because these devices restrict flow of the mixture, loss of calibration is likely due to erosion and/or accumulation of deposits (e.g., of wax, asphaltenes, etc.). These disadvantages may be compounded by poor resolution and accuracy of pressure sensors used to measure the pressure differences. Overcoming the poor resolution and accuracy may require the use of high contraction ratio (e.g., more restrictive) Venturis.
Accordingly, what is needed is an improved method and apparatus for downhole measurement of flow data of a multiphase mixture.